I. Field of the Invention
The present invention relates to a device and a method for reading coded information. The invention also relates to a device for detecting a luminous signal diffused by a support containing coded information.
II. Related Art and Other Considerations
As known, coded information readers (for example, optical readers) capable of locating and decoding the information contained on a support (for example, an optical code associated to an object) which is into a predetermined reading area have been launched on the market in recent years.
In this description and following claims, the expression xe2x80x9ccoded information readerxe2x80x9d refers to any device capable of acquiring information relating to an object (for example distance, volume, size, or its identification data) through the acquisition and processing of a luminous signal diffused by the same object. The expression xe2x80x9ccoded informationxe2x80x9d refers to all identification data contained in an optical code. The expression xe2x80x9coptical codexe2x80x9d refers to any graphic representation having the function of storing a coded information. A particular example of optical code comprises linear or two-dimensional codes wherein the information is coded through suitable combinations of elements having predetermined shape, such as for example squares, rectangles or hexagons, dark-colored (usually black), separate by light elements (spaces, usually white) such as bar codes, stacked codes, and two-dimensional codes in general, color codes, etc. Moreover, the expression xe2x80x9coptical codexe2x80x9d comprises, more in general, also other graphic patterns having function of coding the information, including light printed characters (leffers, numbers, etc.) and particular patterns (such as for example stamps, logos, signatures, digital fingerprints, etc.). The expression xe2x80x9coptical codexe2x80x9d also comprises graphic representations detectable not only in the field of visible light but also in the wavelength range comprised between infrared and ultraviolet.
Only by way of example, and for the purpose of making the following description clearer, explicit reference shall be made to a linear optical code reader (linear reader). Of course, a man skilled in the art shall understand that what said is also applicable to different readers, such as for example two-dimensional optical code readers (matrix or area readers).
Typically, optical code readers comprise an illumination system intended to emit a luminous signal towards the support containing the coded information to be read (optical code) and a reception system intended to pick-up the luminous signal diffused by the illuminated optical code. In particular, said luminous signal is picked up on suitable photo-receiving means (or means for detecting the luminous signal diffused by the illuminated optical code and picked-up through the optical reception system), which in turn generates an electrical signal proportional to the picked-up luminous signal; the electrical signal is intended to be afterwards elaborated and/or processed and decoded, so as to extract the information content.
In a typical embodiment of a conventional linear optical reader, the illumination system comprises an array of LEDs (or more arrays, in matrix or area readers), optionally followed by one or more diaphragms and one or more focalization lenses. Each one of the LEDs of the above mentioned array generates a light beam having a variable luminosity, with a maximum value at the LED optical emission axis, and with decreasing values as the emission angle increases. LEDs are typically aligned so as to be equidistant and parallel to one another and to the reader optical axis. In this way, the light beam exiting from the reader has a luminosity which is variable along the LED alignment direction, with a pattern that is function of the contribution of each LED, of the position of the LED with respect to the reader optical axis, and of the distance of the reader from the illuminated optical code. In fact, it has been noted that, for relatively small distances between reader and code, the profile of the luminous emission beam is not very even along the LED alignment direction and it is possible to distinguish in the profile, at local peaks, the contribution of each LED. As the distance between reader and code increases, besides the decrease of the luminous intensity of the light emission beam, said profile becomes more and more even, and the contribution of each LED becomes less and less clear; said pattern is an intrinsic operation feature of the optical illumination system described above.
The reception system typically comprises one or more lenses and/or diaphragms intended to pick-up, on the photo-receiving means, the light beam diffused by the illuminated code. The system exhibits the feature of transmitting light in a quantitatively different manner depending on whether the light passes through it in the axis, or at the margins of the field of view. In particular, the power per area unit of the luminous beam diffused by the illuminated code and picked-up by the above lenses and/or diaphragms on the photo-receiving means progressively decreases from the center towards the edges of the same beam. This pattern is an intrinsic operation feature of the optical reception system described above.
The photo-receiving means typically comprises an array of photosensitive elements arranged on one (CCD or C-MOS linear sensors) or more parallel lines (CCD or C-MOS matrix sensors). Each photosensitive element is adapted to detect the light portion diffused by a corresponding portion of the illuminated optical code.
From the above, it can be deduced that the illumination and reception systems described above exhibit the similar feature of attenuating the power per area unit of the luminous beam (respectively, of emission and reception) at the margins of their field of view, thus producing a cumulative effect which accentuates the luminous unevenness between the central portion and the margins of the light beam picked-up on the photo-receiving or sensor means. As a consequence, the photosensitive elements at the ends of the sensor (hit by the light coming from the areas at the edges of the optical code) receive less light than those arranged centrally. Since the electrical signal generated by the sensor is proportional to the quantity of light received by the various photosensitive elements, it shall thus have a different pattern in amplitude depending on the distance from the sensor optical axis. This may cause significant problems for the correct operation of the optical reader and, consequently, for the reliability of the reading performed by it. In fact, it would be desirable to obtain, in output from the sensor, an electrical signal with substantially constant amplitude, so as to ensure high precision and reliability standards in the subsequent optical code digitalization and decoding operations.
For the purpose of reducing the undesired effect of attenuation of the power per area unit of the luminous beam hitting on the photosensitive elements of the sensor depending on the distance from the optical axis of the same, various structural solutions have been identified, which have already been used in conventional optical readers. For example, one of said solutions consists in using an illumination system wherein the various emission LEDs are arranged at a non-constant reciprocal distance, and/or with a reciprocal angle. Another solution consists in using a system for controlling the emission LEDs, intended to control the various LEDs in a differentiated way, so as to evenly illuminate the code at the edges as well as at the centre, or illuminate it more at the edges with respect to the centre, so as to compensate the loss of light at the edges caused by the optical reception system.
Thus, in the various structural solutions described above, the drawback mentioned above is at least partly overcome by structurally intervening on (or upstream of) the reader optical illumination system. However, although advantageous for obtaining the expected purposes, such solutions require an ad hoc design of the reader illumination and reception systems (that is, the implementation of a series of structural details in said systems is required) depending on the type of use provided for the reader (in particular, depending on the type of code to be read, the expected distance between reader and code and/or the reader depth of field), thus limiting the possibility of using them for a different use from what they have been designed for. Moreover, such readers are relatively complex from a construction point of view, and this unavoidably affects the production and/or sale cost of the same negatively.
The technical problem at the basis of the present invention is that of identifying a new structural solution which renders feasible, downstream of the sensor, an electrical signal havingxe2x80x94along a predetermined reading directionxe2x80x94any desired and predetermined amplitude pattern. More particularly, an object of the present invention is determining an alternative solution to the known ones, so as to overcome the drawback correlated to the attenuation of the power per area unit of the luminous beam on the photosensitive elements of the sensor when the distance from the optical axis of the same changes (so as to guarantee a correct operation and high reliability of the reader), concurrently overcoming the drawbacks of the solutions used in the prior art, in particular those relating to the critical implementation of the illumination and reception systems of the reader depending on the type of expected use.
Thus, in a first aspect thereof, the present invention relates to a device for reading coded information, comprising:
illumination means;
means for picking up a luminous signal diffused by an illuminated support containing coded information;
means for detecting the luminous signal, intended to generate a first electrical signal proportional to said luminous signal;
means for processing said first electrical signal, intended to extract the coded information content from it; characterised in that it comprises means for varying the amplitude pattern of said first electrical signal along at least one reading direction, so as to generate an electrical signal having, along said reading direction, a predetermined amplitude pattern.
Advantageously, through the device of the present invention it is possible to obtain, downstream of the optical reception path, an electrical signal having, along a reading direction, any desired and/or predetermined amplitude pattern. This is advantageously obtained by suitably varying the amplitude of the electrical signal detected by the photo-receiving means (or detection means, or sensor). In particular, the knowledge of the variation law of the amplitude of the detected electrical signal (which is known in advance, or detectable in output from the sensor) is used to generate, at each scan, a control signal intended to vary the amplitude pattern of the detected electrical signal, so as to generate a resulting signal (in the following description, also referred to as xe2x80x9ccompensatedxe2x80x9d electrical signal) having a desired and predetermined amplitude pattern.
In this description and in the following claims, the expression xe2x80x9cresulting (or compensated) electrical signalxe2x80x9d, refers to an electrical signal wherein the amplitude pattern, along a reading direction, has been varied (through the application of a suitable control signal) so as to assume a predetermined pattern. For example, according to the present invention, the resulting (or compensated) electrical signal can be a signal having a substantially constant amplitude pattern (obtained starting from a detected electrical signal having a variable amplitude), or an electrical signal having an amplitude pattern variable according to a second variation law, for example with a greater amplitude at the ends (obtained starting from a detected electrical signal having a constant amplitude or an amplitude variable according to a first variation lawxe2x80x94for example, having greater amplitude at the centre), etc.
Advantageously, the device of the present invention has a preferred application in those cases in which the electrical signal detected by the photo-receiving means has an amplitude variable, along said at least one reading direction, between at least one maximum value Vmax and at least one minimum value Vmin; in these cases, the means for varying the amplitude pattern of said first electrical signal along said at least one reading direction are, preferably, such as to generate an electrical signal with substantially constant amplitude.
According to the present invention, and unlike the technical solutions described above with reference to the prior art, the drawback correlated to the attenuation of the power per area unit of the luminous beam hitting on the photosensitive elements of the sensor when the distance from the optical axis of the same varies, is advantageously overcome by intervening downstream of the optical reception system of the luminous signal diffused by the illuminated optical code; in particular, said intervention consists in varying the amplitude pattern of the electrical signal detected by the photo-receiving means (or detection means or sensor). Advantageously, this allows to release from the critical implementation of the illumination and reception systems used, as in this case no ad hoc design of said systems is required depending on the expected use of the reader.
In fact, the present invention is based on the fact of assuming the variable pattern of the amplitude of the detected electrical signal (caused by the variation of the power per area unit of the luminous beam diffused by the illuminated optical code) as it is, irrespective of the structural peculiarities of the illumination and reception system used in the device of the invention. In particular, the knowledge of the variation law of the amplitude of the detected electrical signal (known in advance or detectable in output from the sensor) is used to generate, at each scan, a control signal adapted to vary the amplitude pattern of the detected electrical signal, so as to generate an electrical signal havingxe2x80x94along the reading linexe2x80x94a predetermined amplitude pattern, in particular a substantially constant amplitude pattern.
Preferably, the means for varying the amplitude pattern of said first electrical signal operates on said first electrical signal point by point. That is to say, a point by point variation of the detected electrical signal is advantageously carried out; this is particularly advantageous for obtaining a signal having a substantially constant amplitude.
Typically, the illumination means comprises at least one array of LEDs intended to generate a luminous signal. On the other hand, the means for detecting the luminous signal comprises at least one array of photosensitive elements aligned along said at least one reading direction. Preferably, the detection means comprises a CCD or C-MOS, linear or matrix sensor.
Preferably, the means for varying the amplitude pattern of said first electrical signal comprises amplifying means with a gain variable according to a predetermined variation law. Even more preferably, the means for varying the amplitude pattern of said first electrical signal comprises amplifying means with a gain variable according to a variation law which is substantially inverse with respect to the variation law of said first electrical signal (which, as already mentioned, is known in advance or can be detected in output from the sensor) and such as to apply a gain Gmin with value equal to 1 to points of said first electrical signal having maximum amplitude Vmax and a gain Gmax with value equal to Vmax/Vmin to points of said first electrical signal having minimum amplitude Vmin. Advantageously, this allows to obtain, downstream of the optical reception path, an electrical signal with a constant amplitude, equal to Vmax.
Of course, the closer the gain variation law applied to the detected electrical signal is to the inverse of the amplitude pattern of said signal, the more constant is the amplitude pattern of the resulting (or compensated) electrical signal; as a consequence, the subsequent operations of digitalisation and optionally decoding of the optical code will be more precise and reliable. For the purpose of guaranteeing a minimum difference between the gain pattern applied to the detected electrical signal and the inverse of the amplitude pattern of said signal, some criteria (hereinafter called compensation criteria) have been devised, which shall be described in detail in the following of the present description.
Preferably, the device of the invention further comprises means for controlling said variable gain amplifying means. Even more preferably, according to an embodiment of the device of the present invention, said controlling means comprises:
means for generating at least one second electrical signal variable according to an inverse variation law with respect to that of at least one third electrical signal representative of a luminous signal diffused by a reference target placed on at least one predetermined distance;
means for synchronising said at least one second electrical signal with said first electrical signal.
Preferably, said reference target is a white-coloured plane support. In fact, such a target has features that are assimilable to those of a white paper sheet, which is the most common support for printing optical codes. Said target thus ideally represents the plane on which, in the normal operation of the reader, the optical code to be read shall lie. Thus, the electrical signal generated by the sensor in these operating conditions shall contain the characteristic effect of the attenuation of luminosity at the edges of the field of view of the used illumination and reception optical systems. By inverting the pattern of said signal it is obtained how the gain must vary within the scan in order to make effective the intervention of varying the amplitude pattern of the detected signal.
In a preferred embodiment of the device of the present invention, said at least one third signal is representative of the mean of the electrical signals detected, when placing the reference target at different distances. The number of detections carried out at different distances is advantageously defined on the basis of merit evaluations relating to the distance between target and reader, the type of application required for the reader, the heterogeneity of the patterns of the electrical signal detected at the various distances, etc.
In a second aspect thereof, the present invention relates to a device for detecting a luminous signal diffused by a support containing coded information, comprising photo-receiving means intended to generate a first electrical signal proportional to a luminous signal diffused by a support containing coded information, characterised in that it comprises means for varying the amplitude pattern, along at least one reading direction, of said first electrical signal, so as to generate an electrical signal having, along said reading direction, a predetermined amplitude pattern, for example a substantially constant amplitude. Such a device allows to obtain all the advantages mentioned above with reference to the reading device of the present invention, and it is adapted to be used in such reading device.
In a third aspect thereof, the invention relates to a method for reading coded information, comprising the following steps:
illuminating a support containing coded information;
picking-up a luminous signal diffused by said illuminated support;
detecting said luminous signal so as to generate a first electrical signal proportional to said luminous signal;
processing said first electrical signal so as to extract the coded information content from it;
characterised in that it comprises the step of varying the amplitude pattern of said first electrical signal along at least one reading direction, so as to generate an electrical signal having, along said reading direction, a predetermined amplitude pattern.
Such a method can advantageously be implemented through the reading device of the present invention and allows to obtain all the advantages mentioned above.
In particular, advantageously, the method of the present invention has a preferred implementation in those cases in which the electrical signal detected by the photo-receiving means has an amplitude which is variable, along said at least one reading direction, between at least one maximum value Vmax and at least one minimum value Vmin; in these cases, the variation of the amplitude pattern of said electrical signal detected along said at least one reading direction is preferably such as to generate an electrical signal with a substantially constant amplitude. In particular, the method of the invention allows to eliminate the phenomenon of attenuation of the power per area unit of the luminous beam hitting on the photosensitive elements of the sensor when the distance from the optical axis of the same varies, thus guaranteeing highly reliable reads.
Preferably, the variation of the amplitude pattern of said first electrical signal occurs point by point.
Preferably, the step of varying the amplitude pattern of said first electrical signal comprises the step of amplifying said first electrical signal by applying thereto a gain variable according to a predetermined variation law. Even more preferably, the step of varying the amplitude pattern of said first electrical signal comprises the step of amplifying said first electrical signal by applying thereto a gain variable according to a variation law which is substantially inverse with respect to that of said first electrical signal.
Preferably, the step of amplifying said first electrical signal comprises the step of applying a gain Gmin with a value equal to 1 to points of said first electrical signal having maximum amplitude Vmax and a gain Gmax with a value equal to Vmax/Vmin to points of said first electrical signal having minimum amplitude Vmin.
In a first embodiment of the method of the present invention, the step of amplifying said first electrical signal comprises the following steps:
generating at least one second electrical signal variable according to a variation law that is inverse with respect to that of at least one third electrical signal representative of at least one luminous signal diffused by a reference target placed on least at one predetermined distance;
synchronising said at least one second electrical signal with said first electrical signal;
adjusting said gain depending on said at least one second electrical signal.
Preferably, the step of generating said at least one second electrical signal comprises the following steps:
detecting a third signal representative of a luminous signal diffused by an illuminated reference target placed at a predetermined distance;
repeating the previous step for a predetermined number of times, each time varying the distance at which the reference target is placed;
determining at least one fourth electrical signal representative of the electrical signals detected by placing the reference target at different distances;
determining at least one second electrical signal having an inverse variation law with respect to that of said at least one fourth electrical signal.
Preferably, said reference target is a white-coloured plane support.
In an alternative embodiment thereof, the method of the present invention further comprises the steps of carrying out a plurality of consecutive detections, each time amplifying the detected electrical signal by applying thereto a gain variable according to a predetermined variation law, for example an inverse law with respect to that of the previously detected signal.